Search for Chirality in Hydrogenated Magnesium Nanosilicates: A DFT and TD-DFT Investigation

The formation of silicate grains in the interstellar medium (ISM), especially those containing chiral surfaces such as clinopyroxenes, is poorly understood. Moreover, silicate interactions with various forms of hydrogen–proton (H+), neutral H (HI), and molecular hydrogen (H2) are of high importance as hydrogen comprises >90% of the ISM gas budget, and these species play important roles in the formation of new molecules in space. Furthermore, silicate surfaces catalyze the formation of H2 in the interstellar medium formed on dust grain surfaces by H–H association. The technical difficulty of in situ laboratory investigations of nanosilicate nucleation using astrophysically relevant environmental conditions makes computational chemistry a useful tool for studying potential nanosilicate structures. Furthermore, chiral surfaces interacting with chiral organic molecules could serve as templates that lead to the enantiomeric excess of l-amino acids and d-polyols detected in carbonaceous meteorites. However, in order for this effect to take place, an excess of one chiral form of a mineral is required to break the symmetry. This symmetry-breaking event could have been due to the asymmetric absorption of circularly polarized light by the nanosilicate as it traverses star-forming regions. We investigate this possibility using a metastable chiral form of an enstatite dimer as an input for density functional theory (DFT) and time-dependent (TD)-DFT calculations to obtain various properties and circular dichroism spectra. All in all, twenty-six magnesium nanosilicate structures were studied using varying degrees of hydrogenation: none, with HI, with H+, and with H2. The HSE06/aug-cc-pVQZ level of theory was used for the DFT calculations. TD-DFT calculations utilized the CAM-B3LYP/cc-pVQZ and ωB97X-D3/cc-pVQZ functional and basic set pairings. Results show that (1) all twenty-six structures have absorption bands that fall within the 0.6–28.3 μm range available with the newly launched James Webb Space Telescope and (2) there is a small enantioselective effect by UV-CPL on the eight chiral enstatite dimers (predicted g-values of up to 0.007). While the observed effect is small, it opens up the possibility that it is the inorganic material that becomes enantiomerically biased by UV-CPL, driving chiral enhancements in meteoric organic molecules.

HSE06, on the other hand, is a range-separated hybrid functional that includes both short-range and long-range interactions.It has been shown to improve the description of van der Waals interactions and has been demonstrated to perform better than B3LYP in optimizing geometries (2) and calculating energies (3,4) of large molecules and inorganic materials.
Similarly, Tables S. 3 and S. 4 show that calculations utilizing HSE06/aug-cc-pVQZ best match the energy calculations using CCSD(T)+F12.ahigher level of theory.In terms of single-point energy calculations, HSE06 is generally expected to be more accurate than B3LYP (3,4).This is because HSE06 uses a hybrid functional that includes a higher percentage of Hartree-Fock exchange than B3LYP, which typically leads to better accuracy for energy calculations.Additionally, HSE06 includes a correction term to account for long-range electron interactions, which can be particularly important for accurate calculations of large molecules or systems with weak intermolecular interactions.between the HOMO and the LUMO suggests that the system is more prone to electron transfer reactions, useful for photocatalytic reactions as they possess suitable band gaps for absorption of visible light.
The frontier orbitals' energies provide information concerning molecular kinetic stability of the molecules; the larger the HOMO-LUMO gap, generally the more stable and less reactive a molecule appears.For the nanosilicate enstatite studied in this work, there was no consistent trend.In the case of the monomers (Figure 1 from the main text), the most stable molecule has H 2 adsorbed.The monomer with the neutral H and the monomer with the H + have similar predicted stabilities from the SOMO-LUMO and HOMO-LUMO gaps, respectively.However, the HOMO and LUMO orbitals for the protonated monomeric enstatite (Structure C1) are lower in energy compared to that of hydrogenated

Figure S2
The predicted HOMO, LUMO, and their respective energy gap from HSE06/aug-cc-pVQZ for all the different monomer structures (Structures A, B, C, and D), which were optimized using the same level of theory.Green and red orbitals represent the positive and negative regions of the MO, respectively.Each molecular orbital type is show from a bird's eye view, from the side such that the structures are either planar or mostly planar, and from angle such that the silicon atom is closer to the viewer than the magnesium atom.The frontier orbitals for the structures with neutral hydrogen actually represent the SOMO-LUMO gap.
For protonated achiral complexes, the electronic structure for this grouping is lower in energy than the rest.The lower energy HOMO level indicates it is more likely to donate electrons, whereas lower energy LUMO level indicates that the molecule is more likely to accept electrons, implying enhanced reactivity compared with other molecules studied here.Cationic species like this are common in astrochemistry for building up larger molecules, especially molecules that contain H + .The prototypical example is H 3 + , which is responsible for forming the precursors to species observed in dense molecular clouds like CH + and CH 2 + (6-8).Another example of cationic species utilizing hydrogen to build up the size of molecules includes carbon chains (9).
While reaction dynamics were not explored in this study, it would be interesting to investigate the complex formation between the various enstatite nanosilicates and the different forms of hydrogen to determine the size of the reaction barriers, if any, for forming such complexes.Second, the reaction

Figure S3
The predicted HOMO, LUMO, and their respective energy gap from HSE06/aug-cc-pVQZ for all the different achiral structures (Structures E, F, G, and H), which were optimized using the same level of theory.Green and red orbitals represent the positive and negative regions of the MO, respectively.Each molecular orbital type is show from a bird's eye view, from the side such that the structures are either planar or mostly planar, and from angle such that the silicon atom is closer to the viewer than the magnesium atom.The frontier orbitals for the structures with neutral hydrogen actually represent the SOMO-LUMO gap.
dynamics of dimer synthesis has not yet been elucidated.Finally, the reactions between dimers to form even larger complexes (bulk) silicates should be determined.

Figure S4
The predicted HOMO, LUMO, and their respective energy gap from HSE06/aug-cc-pVQZ for all the different chiral structures (Structures I, J, K, and L), which were optimized using the same level of theory.Green and red orbitals represent the positive and negative regions of the MO, respectively.Each molecular orbital type is show from a bird's eye view, from the side such that the structures are either planar or mostly planar, and from angle such that the silicon atom is closer to the viewer than the magnesium atom.The frontier orbitals for the structures with neutral hydrogen actually represent the SOMO-LUMO gap.
Figure S1The placement of adsorbate on the (A) monomer, (B) achiral dimer, and (C) chiral dimer.The top view shows the ring of each species and the bottom view shows the orthogonal view, revealing the planar structures.Positions in red were not calculated due to being equivalent to other calculated positions.Numbers in parentheses represent placement above the plane of the molecule.Oxygen atoms are red, silicon atoms are grey, magnesium atoms are green, and hydrogen atoms are white.